Gould has a discussion of this on page 104. (The following comments do not consider the case where a hairpin continues from a previous staff.)

She says that the hairpin should start on the left side of the first note head, a practice I try to follow, unless, of course, there is a dynamic marking under the notehead, or the hairpin is intended to apply after the note is played.

She also says that the hairpin should end on the right side of the ending note. Strangely enough, her examples show something else: most end on the left side of the noteheads! I think that the ending position of a hairpin can depend on context, but for the general cases of single hairpins, the engraver should decide which way they prefer that hairpins end, that is, left side or right side, and carry this out consistently.

Gould feels that a hairpin would never start before the first note head, or with accidentals before the first note, or before the accidentals. (This assumes that the hairpin in not really starting in the middle of a previous note.) It would never end after a rest that follows the note. I agree, and think that showing an al niente effect by beginning the hairpin before the note or ending after a rest is confusing to musicians and invites questions.

I think that your reply would be more useful as a general advice, John. I seem to have developed some habits of my own, depending on how I want the music performed.

My idiosyncrasies are something like this: About the start of a decrescendo hairpin, I think the 'normal' execution is to let the note sound first, then begin the diminuendo. Starting the diminuendo immediately, or even before the note is sounded as it were, strikes me as more special, like an emphatic 'hai' in an Italian madrigal. The end of a decrescendo hairpin seems to be even more context sensitive (as you imply) – different for piano/percussion and strings/winds for example, and depending on note values and tempo etc.

She also says that the hairpin should end on the right side of the ending note. Strangely enough, her examples show something else: most end on the left side of the noteheads! I think that the ending position of a hairpin can depend on context

Gould says (p. 104): "Good practice is to start the hairpin on the left-hand edge of the note and to finish it on the right-hand edge of a note". About the ends of hairpins especially, I think this is rather too cathegorical, or a rule of thumb with so many exceptions that it is of small value. Most of her examples on pp. 104-105, below HORIZONTAL PLACING, say something different. So, I think that bears out clearly what you, John, say about ends of hairpins depending on context. Common sense rather than narrow rules.